Production of metal powders

ABSTRACT

PRODUCING METAL POSDERS OF CONTROLLED PARTICLE SIZE BY REDUCING OXIDES WITH THE VAPOR OF A REACTIVE METAL IN A VACUUM.

' Ndv. 30, 1971 A. ARIAS 3,623,861

PRODUCTION OF METAL POWDERS Filed Aug. 28. 1969 F|Gv2 FIGJ INVENTOR ALAN ARIAS BY Q ATTORNEYS United States Patent O 3,623,861 PRODUCTION OF METAL POWDERS Alan Arias, Cleveland, Ohio, assignor to the United States of America as represented by the Administrator of the National Aeronautics and Space Administration Filed Aug. 28, 1969, Ser. No. 853,855 Int. Cl. B22f 9/00; C21b 15/02 US. Cl. 75-.5 B 8 Claims ABSTRACT OF THE DISCLOSURE Producing metal powders of controlled particle size by reducing oxides with the vapor of a reactive metal in a vacuum.

ORIGIN OF THE INVENTION The invention described herein was made by an employee of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION Metal powders have been produced by various processes, such as mechanical comminution, atomization of melts, decomposition of metal carbonyls, and hydrogen reduction of oxides and halides. Certain of these processes are time consuming and some are expensive. Also the powders produced by some of these processes are too coarse for their intended use, and some powders have undesirable impurities.

SUMMARY OF THE INVENTION According to the present invention, spaced layers of powdered oxide of the metal or alloy to be produced have pieces of a reactive metal, such as magnesium or lithium, interposed between them in an evacuated container. This container forms the housing of a reactor which is heated to cause the reactive metal to evaporate, and heating is continued until all the oxide has reacted with the reactive metal vapor. The mixture resulting from the reaction includes the powdered metal of the oxide that was reduced and the oxide of the reactive metal. The oxide of the reactive metal is removed from the mixture by leaching. The metal powders remaining after the leaching operation are separated from the leaching solution and washed. The washed metal powders are then dried.

OBJECTS OF THE INVENTION It is, therefore, an object of the present invention to provide a method and apparatus for producing metal powders of controlled particle size and low impurity content.

Another object of the invention is to provide an improved process for the manufacture of metal powders that is less time consuming than previous methods.

A further object of the invention is to provide a process for making metal powders with a minimum amount of waste.

These and other objects of the invention will be apparent from the specification that follows and from the drawings wherein like numerals are used throughout to identify like parts.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a section view of a reactor utilized for pro ducing metal powders in accordance with the preferred form of the invention.

FIG. 2 is a section view of the top portion of a modified reactor forming an alternate embodiment of the invention.

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DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, crucibles 10 are provided for containing a volatile reactive metal 12 as shown in FIG. 1. The crucibles 10 are made of a metal or ceramic that does not react with the volatile reactive metal 12. These crucibles are preferably made of a metal such as stainless steel, Inconel, Nichrome, or nickel.

The volatile reactive metal 12 is in the form of blocks or ingots of magnesium or lithium. These metals will usually melt in the crucibles 10 when heated to the normal reaction temperatures.

The crucibles 10 are placed in a deep container 14 which forms a vacuum tight reactor. The container 14 is preferably of a metal such as stainless steel, Inconel, Nichrome, or nickel.

Layers 16 of the oxide of the metal to be produced are placed between the crucibles 10. This oxide 16 is prevented from coming into direct contact with the reactive metal 12 by screens 18. Each screen 18 is of a material which is preferably the same as that of the crucible 10 or the container 14.

A thin zlayer 20 of the oxide being reduced is also placed in the bottom of the container 14. This layer 20 prevents brazing of the bottom crucible to the container.

Subsequent to the loading of the container 14 in the manner shown in FIG. 1 it is sealed at 22 by electron beam welding. Because electron beam welding is carried out under vacuum, the container 14 will remain under vacuum after sealing. Containers 14 in the form of 2- inch OD, thin-wall, stainless steel tubes have been sealed in this manner.

While electron beam welding is preferred, other methods of vacuum sealing the container 14 may be employed. By way of example, a container 14 without the top cover may be attached to a vacuum pump, and while under vacuum the top portion of the container is deformed by the application of pressure. This flattened portion is then sealed vacuum tight by spot welding or seam welding.

It is preferable to use more reactive metal 12 than is required for stoichiometric reduction of the oxide 16 so that some reactive metal remains unreacted at the end of the reduction process. The excess reactive metal compensates for possible residual gases remaining after evacuation and during sealing.

The alternating stacked arrangement of the reactive metal 12 and the oxides 16 shown in FIG. 1 provides rapid production rates. More particularly, the reaction occurs simultaneously in all the oxide layers 12. Also, the oxide is attacked by the reactive metal vapor from above and below in the intermediate oxide beds.

The sealed container 14 is then moved to a furnace, and the reactants and the container are heated. This vaporizes the reactive metal 12, and the powder or granular oxide in the layers 16 react with the reactive metal vapor. The products of the reaction are mixtures of the metal being produced and the oxide of the reactive metal.

For the reduction of chromic oxide with magnesium vapor the reaction that occurs during heating is:

During the reduction of chromic oxide with lithium vapor the reaction is (2) Cr O +6Li 3Li O+2Cr The reaction during the reduction of nickel oxide with lithium is (3) NiO+2Li- Li 0+Ni Although all of these reactions are exothermic, below certain temperatures they do not provide sufiicient heat to maintain the reaction temperature. However, if the desired reaction temperature is high enough and/or the container 14 is heated fast enough, the reaction becomes self-sustaining. In such a self-sustaining reaction the heat produced by the reaction is greater than the heat lost to the surrounding environment. This reaction may cause the temperature in the container to increase very rapidly.

The particle size of the resulting powder which may be chromium, nickel, or nickel-chromium alloys, is determined by the temperature at which the above reactions are permitted to take place and on the time at temperature. The higher the reaction temperature the larger the particle size of he metal powder, other things being equal. Also the longer the time at temperature the larger the particle size of the metal powder, other things being equal. Thus if the reaction is permitted to become selfsustaining, close control of the particle size of the resulting metal is no longer feasible.

If the temperature of the reaction is kept under control the resulting reaction products are usually in the form of a loose powder which can be treated directly with a leaching solution. However, if the reaction is permitted to become self-sustaining, the products of the reaction usually form agglomerates that must be pulverized before the leaching operation in order to accelerate the leachmg.

After all the oxide has reacted with the metal vapor, the container 14 is removed from the furnace and permitted to cool to ambient temperature. The container 14 is transferred to an argon glove box because the powders produced are usually submicron size and are toxic if inhaled. In the case of nickel powder, processing of the re action products entirely inside an argon glove box also reduces oxidation of the resulting powder.

It is contemplated that the container 14 could be opened in air, and the reaction products also processed in air if proper safety precautions are taken and oxidation is not objectionable. In either case, the container 14 is opened adjacent the top at 24. This enables the container 14 to be reused.

After the products of the reaction are removed from the container 14, the oxide of the reactive metal is removed from the mixture by leaching. The mixture obtained by reaction (1) is treated with dilute nitric acid which dissolves the magnesium oxide but does not attack the chromium powder. The mixture obtained by reaction 2) is leached with distilled water. The mixture obtained by reaction (3) is preferably leached with deaer'ated distilled water.

After the oxide of the reactive metal is leached out the metal powders obtained by the reactions l) and (2) are washed with distilled water. The nickel powders obtained by reaction (3) with dcaerated distilled water. After washing the powders, they are dried by heating either in a vacuum or an inert gas atmosphere.

It is further contemplated that because both chromium oxide and nickel oxide are reducible to the corresponding metal powders with lithium vapor, a mixture or melt of these two oxides will also be reduced by lithium vapor. Therefore the method and apparatus of the invention is likewise applicable to the production of nickel-chrome alloy powders in which two metals are alloyed in any proportions.

DESCRIPTION OF AN ALTERNATE EMBODIMENT Referring now to FIG. 2 there is shown the upper portion of a modified container 14 forming an alternate embodiment of the invention. This container is not sealed at the top as in the case of the reactor shown in FIG. 1.

In this alternate embodiment a cover plate 30 is removably secured to the top of the container 14 in a convenient manner, such as by bolting. The cover plate 30 is provided with a well 32 for receiving a thermocouple 34 for sensing the temperature within the container 14.

A condensing tube 36 extends upward from the cover plate 30. A plurality of slots 38 formed in the end of the tube 36 facilitate the evacuation of the container 14. A dust cap 40 is positioned on the end of the tube 36 adja cent the slot 38.

In operation the container 14 shown in FIG. 2 is loaded in the same manner as the container 14 shown in FIG. 1. The cover plate 30 is then positioned on the open end of the container.

The loaded container 14 with the cover plate 30 in place is placed in a vacuum furnace. After sealing the vacuum furnace the evacuation process is started. While the furnace is being evacuated the container is likewise evacuated through the tube 36 and slots 38.

EXAMPLES The invention will be better understood by reference to the following examples:

Example 1 A container 14 was loaded with a total of 160 grams of chromic oxide (Cr O powder and a total of grams of magnesium positioned in five alternating layers in the manner shown in FIG. 1. The container 14 was sealed in an electron beam welder as previously described. The sealed evacuated container was then placed in a furnace.

The sealed container 14 was heated slowly to 770 C., and it was held at this temperature for 37 hours. The sealed container 14 was removed from the furnace and permitted to reach ambient temperature. The loaded container 14 was transported to an argon glove box, opened adjacent the top with a tube cutter, and the reaction products were removed.

A black powder mixture resulted from the reaction. This powder was treated with enough dilute nitric acid to dissolve the magnesium oxide. A dark grey powder remained after centrifuging the resulting slurry, and this powder was washed several times with distilled water. In each of these washings the powder was separated from the water by centrifuging. Some of the chromium powder in an amount of about 5 weight percent was lost as colloidal suspension in the washes. The chromium powder was then dried by heating in a vacuum.

The chromium powder so obtained had an average particle size of 0.15 micron. A chemical analysis showed the powder to contain 0.8 weight percent oxygen, 262 p.p.m. (parts per million) residual magnesium, 148 p.p.m. nitrogen, 184 p.p.m. carbon, and 388 p.p.m. sulfur.

Example 2 A container 14 was loaded with powdered chromium oxide and lithium. The lithium was about 10 percent in excess of stoichiometric requirements, and the total amount of chromic oxide used was about 40 grams in five layers. The container 14 was sealed as in Example 1 and placed in a furnace. The container was heated slowly to 600 C. and held at this temperature for 12 hours.

The container was removed from the furnace, allowed to cool down to ambient temperature, and transported to an argon glove box. The reaction products were removed from the reactor. The black powder mixture resulting from the reaction was treated with distilled water in order to leach out the lithium oxide in the mixture. The powder was separated from the liquid by decantation. The chromium powder was further washed with distilled water in the same manner. The recovered chromium powder was then dried by heating it in vacuum. This chromium powder had an average particle size of 0.134 micron. A chemical analysis showed it to contain 0.678 weight percent oxygen and 0.007 weight percent residual lithium.

Example 3 As in Example 2, chromic oxide was reduced with lithium vapor, except that the reactor was heated at 800 C. for 12 hours at temperature. The resulting mixture was leached, washed, and dried as in Example 2. The resulting chromium powder had an average particle size of 0.6 micron and analyzed 0.176 weight percent oxygen and 0.015 weight percent residual oxygen.

Example 4 A container 14 was loaded with nickel oxide powder and lithium metal. The lithium weighed about 10 percent in excess of stoichiometric requirements. The nickel oxide powder was placed in five layers as shown in FIG. 1.

The container was sealed by electron beam Welding as previously described. The sealed evacuated container was placed in a furnace. The container was heated slowly to 600 C. and held at this temperature for 12 hours. The container was then removed from the furnace, allowed to cool down to ambient temperature, and transported to an argon glove box. The reaction products Were then removed from the container. The resulting powder mixture was treated with deaerated distilled water in order to leach out the lithium oxide, and the nickel powder was recovered by decantation. The nickel powder was washed several times with deaerated distilled water, the nickel powder again recovered by decantation. The nickel powder was then dried by heating it in a vacuum.

The average particle size of this nickel powder was 1.25 microns. A chemical analysis showed it to contain 0.114 weight percent oxygen and 0.080 weight percent residual lithium.

Example A container 14 was loaded and sealed as in the previous examples. This container was heated quite rapidly at a rate of about 30 C. per minute in an induction heated furnace in which the reactor itself was the susceptor. The temperature of the reactor was monitored with an optical pyrometer.

At about 850 C. and without an increase in power input to the furnace the temperature of the reactor increased to about 1200" C. in about one minute. It then cooled down to about 1000 C. indicating that a self-sustaining reaction had taken place.

After a total time of less than 30 minutes above 850 C., the container was cooled down to ambient temperature and then the reaction products recovered as in Example 1. The reaction mixture obtained had light grey patches and formed cinder-like pieces. These pieces were converted into a powder by ball milling in water.

The chromium was recovered as in Example 1. The powder obtained was light grey in color and had some individual chromium powder particles visible with the unaided eye.

Although the temperature at which the self-sustaining reaction occurred was about 850 C., in Example 4 it should be noted that in other cases it may be different. This is because the reaction rates will depend upon the particle size of the oxide being reduced, and the heat losses will depend upon the size and shape of the reactor, the bulk density of the powder being reduced, and the heating rate.

What is claimed is:

1. A method of making metal powders of controlled particle size comprising the steps of loading alternate layers of a reactive metal and a powdered metal oxide into a container, the amount of reactive metal in said container being in excess of that required for stoichiometric reduction of the metal oxide,

evacuating said container,

heating the alternate layers to a temperature sufficient to vaporize the reactive metal and below that at which a self sustaining reaction occurs whereby the reactive metal vapor reacts with the metal oxide to form a mixture of reactive metal oxide and metal powder, maintaining said alternate layers at said temperature for a time sufficient to effect the complete reaction of the metal oxide with the reactive metal vapors,

cooling the resulting mixture to ambient temperature,

and

recovering the metal powder from the mixture.

2. A method of making metal powders as claimed in claim 1 wherein the powdered metal oxide is selected from the group consisting of chromium oxide and nickel oxide.

3. A method of making metal powders as claimed in claim 1 wherein powder of both chromium oxide and nickel oxide are loaded into the container.

4. A method of making metal powders as claimed in claim 1 wherein the reactive metal is selected from the group consisting of lithium and magnesium.

5. A method of making metal powders as claimed in claim 1 including the step of leaching the reacted mixture to recover the metal powders.

6. A method of making metal powders as claimed in claim 1 including the step of sealing the container under vacuum prior to heating.

7. A method of making nickel powders of controlled particle size comprising the steps of loading alternate layers of lithium and powdered nickel oxide into a container, the amount of lithium in said container being in excess of that required for stoichiometric reduction of the nickel oxide,

placing the loaded container in a vacuum environment,

sealing said loaded container in said vacuum environment,

placing said sealed loaded container in a furnace,

heating the alternate layers to a temperature sufficient to vaporize the lithium and below that at which a self sustaining reaction occurs whereby the lithium vapor reacts with a nickel oxide to form a mixture of lithium oxide, and nickel.

maintaining said alternate layers at said temperature for a time suflicient to effect the complete reaction of the nickel oxide with the lithium vapors,

cooling the resulting mixture to ambient temperature,

and

recovering the nickel powder from the mixture.

8. A method of making nickel powders as claimed in claim 1 including sealing the loaded container in a vacuum environment by electron beam Welding.

References Cited UNITED STATES PATENTS 3,053,649 9/1962 Galmiche 0.5 B 908,154 12/1908 Seward et a1 750.5 B 1,290,181 l/1919 Hall 750.5 B 1,373,038 3/1921 Weber 750.5 B 2,848,324 8/1958 Krapf 7527 2,919,189 12/1959 Nossen et a1 75-27 3,194,649 7/1965 OkaZaki 7527 FOREIGN PATENTS 762,652 12/1956 Great Britain 75-27 986,808 3/1965 Great Britain 75-27 L. DEWAYNE RUTLEDGE, Primary 'Examiner W. W. STALLARD, Assistant Examiner US. Cl. X.R. 75-0.5 BA, 27 

